1. Field of the Invention
The present invention relates generally to semiconductor wafer processing techniques using chemical-mechanical polishing, and more particularly to methods and apparatus for measuring the removal of material from a polishing pad.
2. Description of Related Art
During the manufacture of integrated circuits it is necessary to polish a thin wafer of semiconductor material in order to remove material and dirt from the wafer surface. Typically, a wet chemical abrasive or slurry is applied to a motor driven polishing pad while a semiconductor wafer is pressed against it in a process well known in the prior art as chemical-mechanical polishing (CMP). The polishing platen is usually covered with a soft wetted material such as blown polyurethane. The polishing effects on the wafer result from both the chemical and mechanical action.
The polishing pad contacts the wafer surface while both wafer and pad are rotating on different axes. The rotation facilitates the transport of the abrasive containing polishing slurry between the pad and the wafer. The choice of polishing pad and slurry is determined by the material being polished, and the desired flatness of the polished surface.
Apparatus for polishing thin flat semiconductor wafers are well known in the art. U.S. Pat. Nos. 4,193,226 and 4,811,522 to Gill, Jr. and U.S. Pat. No. 3,841,031 to Walsh, for instance, disclose such apparatus.
The condition of the polishing pad directly affects the polishing rate of material removal and uniformity of removal from the semiconductor wafer. The material of the polishing pad is also chosen for its ability to act as a carrier of the slurry and to wipe away the grit and debris resulting from the polishing action.
The hardness of the pad has a strong influence on wafer flatness. The use of hard and less compressible pads made of urethane materials, e.g., IC series pads from Rodel, Inc., results in surfaces with reduced topography when compared to pads made from urethane foam, e.g., Politex pads from Rodel, Inc., or felt polishing pads.
However, urethane pads have a porous structure throughout. During polishing, slurry can accumulate in the pore structure. This diminishes the polishing removal rate and degrades the polishing removal uniformity. To reduce these effects, the pores on the pad surface may be opened or a fresh pad surface exposed. These processes are commonly referred to as pad conditioning.
Conditioning may take place during or after the polishing process. The most common method of pad conditioning is a mechanical abrasion of the pad surface. Materials such as steel blades or abrasive wheels are often used. While conditioning of the pad surface improves polishing uniformity and rates, it has the detrimental effect of removing a quantity of pad material.
This presents the problem encountered in the process of pad conditioning, and chiefly addressed by the present invention: the unregulated, non-uniform removal of pad material. If the abrasion of the pad material is not uniform across the pad surface that contacts the wafer, the polishing uniformity and the pad's useful lifetime will be adversely affected. The ideal pad surface after conditioning should be flat (no curvature within the conditioned area).
Additionally, there are occasions when the semiconductor wafer, secured in the wafer carrier during polishing, is dislodged from the wafer carrier. When this occurs, the polishing process subjects the unrestrained wafer to damage. A method to monitor pad degradation that includes detecting the presence of a dislodged semiconductor wafer would enhance the effectiveness and efficiency of the polishing process.
Presently, the only ways available to measure the pad material removal are destructive to the pad; cutting a piece from the pad and using a micrometer to measure thickness, or contacting the pad using a micrometer and a straightedge across the pad surface. Thus, pad destruction or pad contamination may result from measurements currently made in the prior art.
Predominantly, the effectiveness of the CMP process has been monitored by measuring the degree of planarization of the semiconductor wafer itself. End point detection schemes have been enacted to monitor the removal of material on a semiconductor substrate without removing the devices formed underneath the material. Typically, this planarization process is accomplished by control of the rotational speed, downward pressure, chemical slurry, and time of polishing of the CMP process.
A number of optical and other methods exist for determining when polishing endpoint on the semiconductor wafer has occurred. These methods monitor the wafer itself and include acoustical wave generation and detection, thermal imaging, friction sensing, impedance or capacitance measurements, monitoring the current of the motor used to rotate the wafer against a polishing pad, stylus profilometry, phase shift interferometry, light scattering analysis, scanning tunneling microscopy, and three dimensional optical profiling.
U.S. Pat. No. 5,081,796, issued to Schultz on Jan. 21, 1992, entitled, "METHOD AND APPARATUS FOR MECHANICAL PLANARIZATION AND ENDPOINT DETECTION OF A SEMICONDUCTOR WAFER", teaches a laser interferometer measuring device employed to detect the thickness of a material being planarized. However, the non-uniformity of polishing pad material removal is not addressed, as the polishing pad thickness or its relative change is not measured or monitored.
U.S. Pat. No. 5,222,329, issued to Yu on Jun. 29, 1993, entitled, "ACOUSTICAL METHOD AND SYSTEM FOR DETECTING AND CONTROLLING CHEMICAL-MECHANICAL POLISHING (CMP) DEPTHS INTO LAYERS OF CONDUCTORS, SEMICONDUCTORS, AND DIELECTRIC MATERIALS", teaches a method for sensing acoustical waves generated when the depth of material removal on the semiconductor substrate reaches a certain determinable distance from the interface and generates specifically defined detection signals. However, this passive acoustical technique deals with a method of polishing and monitoring for end point detection on the semiconductor wafer, not a method for monitoring or measuring a change in the polishing pad thickness. Nor does this technique employ an active radiation source in its measurement scheme.
U.S. Pat. No. 5,461,007, issued to Kobayashi on Oct. 24, 1995, entitled, "PROCESS FOR POLISHING AND ANALYZING A LAYER OVER A PATTERNED SEMICONDUCTOR SUBSTRATE", teaches a reflected radiation beam or radiation scattering analyzer approach to monitoring the polishing layer over a previously patterned semiconductor substrate. A detector analyzes the reflected beam to determine a detected intensity. The reflected beam's intensity is a function of the reflection angle which correlates to different layer depths on the substrate. Thus, it may correlate to when a polishing end point has been reached. Although the general measurement technique utilizes reflected radiation, beam intensity is the sole operational parameter, not an interferometer technique utilizing phase change or time delay. Also, once again, the polishing pad itself is not monitored or measured in this prior art.
As shown from the prior art, the condition of the polishing pad surface, although adversely affecting planarization, is not an operational parameter that has been monitored or measured. Consequently, although the effects of a degraded polishing pad have been acutely addressed by analyzing the semiconductor wafer, an approach to monitor the polishing pad itself and measure the particle removal as a function of pad surface depth is novel to this invention.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for monitoring and measuring the diminished thickness in a polishing pad.
It is another object of the present invention to provide a method and apparatus of the type described which operates non-intrusively to the chemical-mechanical polishing process, not relying on physical contact as a method for layer removal monitoring.
A further object of the present invention is to provide a method and apparatus of the type described which utilizes an active measurement system with a radiated energy source and detector to monitor and measure the diminished thickness of a polishing pad.
It is another object of the present invention to provide a method and apparatus of the type described which utilizes an ultrasonic or laser interferometer measurement system to monitor and measure the diminished thickness of a polishing pad.
A further object of the invention is to provide a method and apparatus of the type described which monitors and measures, in situ, the relative change in thickness of the polishing pad.
It is another object of the present invention to provide a method and apparatus of the type described which utilizes a closed-loop feedback process to control the chemical-mechanical polishing in real time in order to minimize the degradation in polishing pad uniformity.
It is yet another object of the present invention to provide a method and apparatus of the type described which can detect the release of a semiconductor wafer from the chemical-mechanical polishing tool wafer carrier during polishing.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.